Method of operating a reaction propulsion engine and fuels therefor

ABSTRACT

1. The method of operating a ram air reaction propulsion system comprising: directing a fuel capable of endothermically dissociating at temperatures between about 200* to 2,000* F. into indirect heat exchange between the inlet ram air of the system and said fuel to bring about the endothermic dissociation of at least a portion of the fuel by the transfer, prior to mechanical compression of the ram air, of a portion of the heat energy from the ram air of the system to the fuel prior to combustion of the fuel in the ram air; utilizing a portion of the ram air heated fuel to mechanically compress the fuel cooled ram air; burning the fuel exhausting from said further air compressing step in the said further compressed air; and thereafter expanding the combustion products through an outlet nozzle of the system.

llnited States Patent Wolf at al.

[54] METHOD OF OPERATING A REACTION PROPULSION ENGINE AND FUELS THEREFORInventors: Robert L. Wolf, Chesterfield County; Christopher J. Cowlin,Richmond, both of Va.

Assignee: Texaco Inc., New York, NY.

Filed: June 1, 1966 Appl. No.: 554,597

Related US. Application Data Continuation of Ser. No. 325,352, Nov. 20,1963, abandoned, which is a continuation-inpart ofSer. No. 152,097,Nov.13,1961.

References Cited UNITED STATES PATENTS 12/1952 Phanery ..60/5.6

TURBINE BY- PASS VA LVE AIR r me rueuqet TURBINE TEMPERATURE SPEEDSENSING MEANS 1 Sept. 12, 1972 3,067,594 12/1962 Bland ..60/35.6

Primary ExaminerSamuel Feinberg Attorney-Stowell and Stowell EXEMPLARYCLAIM 1. The method of operating a ram air reaction propulsion systemcomprising: directing a fuel capable of endothermically dissociating attemperatures between about 200 to 2,000 F. into indirect heat exchangebetween the inlet ram air of the system and said fuel to bring about theendothermic dissociation of at least a portion of the fuel by thetransfer, prior to mechanical compression of the ram air, of a portionof the heat energy from the ram air of the system to the fuel prior tocombustion of the fuel in the ram air; utilizing a portion of the ramair heated fuel to mechanically compress the fuel cooled ram air;burning the fuel exhausting from said further air compressing step inthe said further compressed air; and thereafter expanding the combustionproducts through an outlet nozzle of the system.

3 Claims, 2 Drawing Figures o znp sso JFfiZUMP TURBINE COMBUSTOR NOZZLE-I COMPRESSOR SEQkT JRE F SENSING MEANS TANK METHOD OF OPERATING AREACTION PROPULSION ENGINE AND FUELS THEREFOR This application is acontinuation application of my application, Ser. No. 325,352, filed Nov.20, 1963 now abandoned, and application, Ser. No. 325,352 is acontinuation-in-part of our co-pending application, Ser. No. 152,097, R.L. Wolfet al., filed Nov. 13, 1961.

This invention relates to reaction propulsion engines and to method ofoperating such engines. In one particular aspect, the present inventionrelates to fuels suitable for use in air-breathing reaction propulsionengines capable of operation at hypersonic speeds.

The use of rocket propulsion is generally inefficient because of theinherently low fuel impulse value of chemical rocket systems. The usualram jet cycles are unattractive at hypersonic speeds because at speedsabove about Mach 8 the amount of energy at the air inlet is so greatthat the additional heat liberated by combustion of the fuel in the ramair is mainly expended in dissociating the components of the combustiongases. The energy absorbed by such fuel or combustion productdissociation is only partially recovered on recombination when the gasesare expanded through the exit nozzle to the atmosphere.

These difficulties, in the ram jet cycles, can be avoided bytransferring a portion of the energy of the inlet air to the fuel byheat exchange between the inlet air and the fuel. The energy thustransferred is utilized at least in part by expanding the fuel in animpulse reaction nozzle and/or a turbine driving an air compressor.

In general, the invention comprises the method of operating anair-breathing propulsion system comprising transferring a portion ofenergy from the inlet air of the system to the fuel supply by indirectheat exchange between the inlet air and a fuel capable ofendothermically dissociating at temperatures between about 200 to 2,000E, converting the useful work at least a portion of the heat transferredto the said fuel by direct expansion of at least a portion of the fuel,burning at least a portion of the said fuel in the inlet air, andexpanding the combustion products through an impulse expansion nozzle.

The invention will be more particularly described with reference to thedrawings wherein:

FIG. 1 is a schematic representation of the operation of such a systemat high Mach numbers; and

FIG. 2 is a schematic representation of a system of the invention atlower Mach numbers.

Referring to FIG. 1 of the drawings, fuel is pumped from the fuel tankthrough an indirect heat exchanger wherein its temperature is raised byheat exchange with the hot incoming ram air.

The rate of flow of fuel to the ram air heat exchanger may be controlledwithin wide limits by varying the output from the fuel pump or byproviding a control valve in the outlet line from the fuel pump to theheat exchanger. The pump output volume or the control valve may bemanually controlled and/or as indicated in the drawing, the control ofthe pump output or control for an output control valve may be providedby the compressor outlet temperature sensing means which would insurethat sufficient fuel is passed to the heat exchanger to maintain limitson the turbine inlet temperature and the compressor dischargetemperature.

The heated fuel passes through a turbine bypass valve where only theamount of heated fuel necessary to operate the turbine is actuallypassed through the turbine. The turbine operates the pump which passesthe fuel from the fuel storage tank to the heat exchanger and also theturbine drives the air compressor which compresses the air after it hasleft the heat exchanger prior to passage of the air to the combustor.The heated fuel not required for turbine operation may be passeddirectly to the combustor where it is burned, along with the fuelexhausting from the turbine, with the air from the air compressor. Thecombustion gases are passed to the atmosphere through the expansionnozzle as indicated in the drawing.

It will also be appreciated that the fuel passing directly from theturbine bypass valve to the combustor may be expanded through a thrustnozzle positioned within the combustor or all or a portion of this fuelmay be expanded through a thrust nozzle external of the combustor. Theuse of such a thrust nozzle has particular utility where the amount offuel needed to cool the ram air is greater than the amount which couldbe burned stoichiometrically with the available air supply.

The turbine bypass valve may be preset for the particular mission of thevehicle to be driven by the reaction engine or, as shown in FIG. '1, thebypass valve may be provided with a controller 10. The controller 10 mayregulate the turbine bypass valve in accordance with the temperature ofthe fuel passing to the turbine or by sensing the speed of the turbine,thereby preventing overheating and/or overspeeding of the turbine oroverspeeding of the air compressor driven thereby. Further, thecontroller for the turbine bypass valve may be fuel or compressortemperature responsive, vehicle altitude or speed responsive, or acombination of two or more of these factors.

Operation of the system of the invention at low Mach numbers isschematically represented in Flg. 2. Operation is similar to that shownin FIG. 1 except that a regenerative heat exchanger is included in thefuel heating cycle to provide additional heat for the fuel for operationof the turbine. As in thesystem illustrated in FIG. 1, the fuel ispumped to the heat exchanger and its temperature raised by heat exchangewith the incoming ram air. The heated fuel is then passed to the turbinebypass valve and sufficient fuel is diverted for use in operation of theturbine. Interposed between the turbine bypass valve and the turbine isa temperature modulating valve and a mixer. A portion of the fuel heatedby means of the incoming air is further heated by the regenerative heatexchange in the combustion zone and passed to the mixer wherein it isreturned to the body of heated fuel to be expanded through the turbine.It is the function of the turbine temperature modulating valve to passsufiicient fuel to the regenerative heat exchanger to provide that thefuel mixture finally expanded through the turbine has a high enoughenergy value to satisfy the requirements of the turbine in operating thefuel pump and the air compressor. The heated fuel, together with thefuel exhausting from the turbine, may be combined with the compressedair and are burned in the combustor and passed through a nozzle to theatmosphere as before.

As discussed with reference to the embodiment shown in FIG. 1, the rateof flow of fuel to the ram air heat exchanger may be controlled withinwide limits by varying the output from the fuel pump or by providing acontrol valve in the outlet line from the fuel pump to the heatexchanger. The pump output volume or the control valve may be manuallycontrolled and/or as indicated in the drawing, the control of the pumpoutput or control for an output control valve may be provided by thecompressor outlet temperature sensing means which would insure thatsufficient fuel is passed to the heat exchanger to maintain limits onthe turbine inlet temperature and the compressor discharge temperature.

For high cooling efficiency and to provide a high absorption of energyper pound of fuel, it is desirable that the fuel selected have a highheat capacity within the expected operating temperature range of 200 F.to about 2,000 F. At the same time, for efficient conversion of energyinto thrust in the expansion process, the fuel should be one whichprovides low average molecular weight components in the exhaust gases.It has been discovered that fuels which undergo endothermicdecomposition or dissociation at temperatures which will provide maximumcooling of the inlet air are particularly suited for use in such asystem. The most suitable fuels will dissociate at these temperatures tohydrogen and other relatively low molecular weight compounds without theformation of free carbon particles. Preferred examples of such fuels areammonia, methyl alcohol, ethylene glycol, and cyclohexane.

The preferred high heat capacity fuels, ammonia, methyl alcohol,ethylene glycol and cyclohexane, dissociate into hydrogen and nitrogen,hydrogen and carbon monoxide, hydrogen and carbon monoxide, and hydrogenand benzene, respectively. The dissociation is endothermic and theresulting gaseous products are of low average molecular weight and areexceptionally clean; that is, they and their combustion products havelittle or no tendency to foul the engine as they contain no free carbon.

When such fuels are used at relatively low flight speeds below aboutMach 1.5 in a system where the incoming air and a regenerative heatexchanger are used to heat the fuel prior to combustion, there will bevery little heating of the fuel by the incoming air. Most of the heatrequired to decompose and/or evaporate the fuel and heat the fuel to theturbine inlet temperature required to operate the air compressor willcome from the regenerative heat exchanger. As the flight speed and thetemperature of the inlet air increase, there will be more cooling of theair ahead of the compressor and hence more heating of the fuel prior toits passage to the turbine and/or combustion zone. Thus, less heat willbe required from the combustion zone via the regenerative heat exchangerand less fuel will be programmed through the regenerative heat exchangerlocated in the combustion area prior to its expansion across theturbine. At still higher flight speeds, all of the required heat willcome from the cooling of the inlet air and no heat will be taken fromthe combustion zone via the regenerative heat exchanger.

The air to fuel indirect heat exchanger located ahead of the aircompressor serves three main purposes. The first is to increase theavailable turbine work of the fuel by heat addition without combustionwhile lowering the work required to compress the air thereby making thecycle more efficient. The second is to cool the incoming air toacceptable temperature levels, preferably below 1,200 F., to avoidoverheating the compressor. The third is to increase the density of theair through cooling to give a higher mass flow per unit compressorfrontal area and a resultant higher level of thrust.

At higher flight Mach numbers, it becomes increasingly important to coolthe incoming air, down to acceptable levels for the compressor. It thenbecomes necessary to run the heat exchanger richer than at lower Machnumbers, i.e., using an amount of fuel in excess of that which can beexpanded across the turbine without overspeeding the rotating assembly.The excess fuel is passed either directly to the combustor or to athrust nozzle mounted in the combustor for combustion along with thefuel that has been expanded across the turbine. A portion of the fuelused in cooling the inlet air may be burned stoichiometrically with theair in the combustor while the remainder may be expanded directly to theatmosphere through a separate thrust nozzle, not shown in the drawings.The proportion separately expanded is determined by the maximum exhaustgas temperature and the maximum degree of dissociation to be maintained.The use of endothermically dissociating fuels according to the processof our invention minimizes the expenditure of uncombusted fuel.

EXAMPLE I An engine of the type illustrated in FIG. 1 can be operatedefficiently through the velocity range from about Mach 4 to about Mach10 at altitudes up to 150,000 feet using the endothermicallydissociating fuels of the invention while holding the compressor inlettemperature below l,200 F. by heat exchange between the inlet air andthe fuel.

EXAMPLE II An engine of the type illustrated in FIG. 2 can be operatedefficiently through the velocity range from static launch to Mach l0 andat altitudes up to 150,000 feet using the endothermically dissociatingfuels of the invention. Below about Mach 4 heat is added to the fuel inthe regenerative heat exchanger to provide enough energy to drive theturbine without the use of auxiliary fuel combustion ahead of theturbine. Above about Mach 6 none of the fuel is passed to theregenerative heat exchanger as inlet air heating of the fuel providesall of the energy requirement of the turbine.

We claim:

1. The method of operating a ram air reaction propulsion systemcomprising: directing a fuel capable of endothermically dissociating attemperatures between about 200 to 2,000 F. into indirect heat exchangebetween the inlet ram air of the system and said fuel to bring about theendothermic dissociation of at least a portion of the fuel by thetransfer, prior to mechanical compression of the ram air of a portion ofthe heat energy from the ram air of the system to the fuel prior tocombustion of the fuel in the ram air; utilizing a portion of the ramair heated fuel to mechanically compress the fuel cooled ram air;burning the fuel exhausting from said further air compressing step inthe said further compressed air; and thereafter expanding the combustionproducts through an outlet nozzle of the system.

2. The method defined in claim 1 wherein the ram air is furthercompressed by expanding the ram air heated fuel across a turbine of aturbocompressor. 5 3. The inyention defined in claim 2 wherein the fuelis selected from the group consisting of ammonia, methyl alcohol,ethylene glycol and cyclohexane.

1. The method of operating a ram air reaction propulsion system comprising: directing a fuel capable of endothermically dissociating at temperatures between about 200* to 2,000* F. into indirect heat exchange between the inlet ram air of the system and said fuel to bring about the endothermic dissociation of at least a portion of the fuel by the transfer, prior to mechanical compression of the ram air of a portion of the heat energy from the ram air of the system to the fuel prior to combustion of the fuel in the ram air; utilizing a portion of the ram air heated fuel to mechanically compress the fuel cooled ram air; burning the fuel exhausting from said further air compressing step in the said further compressed air; and thereafter expanding the combustion products through an outlet nozzle of the system.
 2. The method defined in claim 1 wherein the ram air is further compressed by expanding the ram air heated fuel across a turbine of a turbocompressor.
 3. The invention defined in claim 2 wherein the fuel is selected from the group consisting of ammonia, methyl alcohol, ethylene glycol and cyclohexane. 